Method for forming transparent conductive oxide

ABSTRACT

Embodiments disclosed herein generally relate to a process of depositing a transparent conductive oxide layer over a substrate. The transparent oxide layer is sometimes deposited onto a substrate for later use in a solar cell device. The transparent conductive oxide layer may be deposited by a “cold” sputtering process. In other words, during the sputtering process, a plasma is ignited in the processing chamber which naturally heats the substrate. No additional heat is provided to the substrate during deposition such as from the susceptor. After the transparent conductive oxide layer is deposited, the substrate may be annealed and etched, in either order, to texture the transparent conductive oxide layer. In order to tailor the shape of the texturing, different wet etch chemistries may be utilized. The different etch chemistries may be used to shape the surface of the transparent conductive oxide and the etch rate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent ApplicationSer. No. 61/185,172 (APPM/14442L), filed Jun. 8, 2009 and U.S.Provisional Patent Application Ser. No. 61/186,636 (APPM/14442L02),filed Jun. 12, 2009, each of which are herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments disclosed herein generally relate to a process of depositinga transparent conductive oxide layer over a substrate.

2. Description of the Related Art

Photovoltaic (PV) devices or solar cells are devices which convertsunlight into direct current (DC) electrical power. PV or solar cellstypically have one or more p-n junctions. Each junction comprises twodifferent regions within a semiconductor material where one side isdenoted as the p-type region and the other as the n-type region. Whenthe p-n junction of the PV cell is exposed to sunlight (consisting ofenergy from photons), the sunlight is directly converted to electricitythrough the PV effect. PV solar cells generate a specific amount ofelectric power and cells are tiled into modules sized to deliver thedesired amount of system power. PV modules are created by connecting anumber of PV solar cells and are then joined into panels with frames andconnectors.

Several types of silicon films, including microcrystalline silicon(μc-Si), amorphous silicon (a-Si), polycrystalline silicon (poly-Si) andthe like, may be utilized to form PV devices. A transparent conductivefilm, sometimes referred to as a transparent conductive oxide (TCO) maybe used as a top surface electrode disposed on the top of the PV solarcells. Furthermore, the TCO layer may be disposed between a substrateand a photoelectric conversion unit as a contact layer. The TCO shouldhave high optical transmittance in the visible or higher wavelengthregion to facilitate transmitting sunlight into the solar cells withoutadversely absorbing or reflecting light energy. Additionally, lowcontact resistance and high electrical conductivity of the TCO aredesired to provide high photoelectric conversion efficiency andelectricity collection. A certain degree of texture or surface roughnessof the TCO is also desired to assist sunlight trapping in the films bypromoting light scattering. Overly high impurities or contaminant of theTCO often result in high contact resistance at the interface of the TCOand adjacent films, thereby reducing carrier mobility within the PVcells. Furthermore, insufficient transparency of the TCO may adverselyreflect light back to the environment, resulting in a diminished amountof sunlight entering the PV cells and a reduction in the photoelectricconversion efficiency.

Therefore, there is a need for an improved method for fabricating a TCO.

SUMMARY OF THE INVENTION

Embodiments disclosed herein generally relate to a process of depositinga transparent conductive oxide layer over a substrate. The transparentoxide layer is sometimes deposited onto a substrate for later use in asolar cell device. The transparent conductive oxide layer may bedeposited by a “cold” sputtering process. In other words, during thesputtering process, a plasma is ignited in the processing chamber whichnaturally heats the substrate. No additional heat is provided to thesubstrate during deposition such as from the susceptor. After thetransparent conductive oxide layer is deposited, the substrate may beannealed and etched, in either order, to texture the transparentconductive oxide layer. In order to tailor the shape of the texturing,different wet etch chemistries may be utilized. The different etchchemistries may be used to shape the surface of the transparentconductive oxide and the etch rate.

In one embodiment, a method is disclosed. The method includes forming atransparent conductive oxide layer over a substrate and etching theformed transparent conductive oxide layer to form a roughened surface.The etching comprising exposing the formed transparent conductive oxidelayer to a wet etchant composition selected from the group consistingof: HCl, H₂O₂ and de-ionized water having a concentration ratio of HCland H₂O₂ relative to the de-ionized water of about 0.25 percent to about10 percent; HCl and de-ionized water having a concentration ratio of HClrelative to the de-ionized water of about 0.25 percent to about 10percent; HNO₃ and de-ionized water having a concentration ratio of HNO₃relative to the de-ionized water of about 0.25 percent to about 10percent; or H₂SO₄ and de-ionized water having a concentration ratio ofH₂SO₄ relative to the de-ionized water of about 0.25 percent to about 10percent. The method also includes annealing the etched transparentconductive oxide layer.

In another embodiment, a method is disclosed. The method includessputter depositing a transparent conductive oxide layer over a substrateand etching the sputter deposited transparent conductive oxide layer toform a roughened surface. The etching comprises exposing the sputterdeposited transparent conductive oxide layer to a wet etchantcomposition selected from the group consisting of: a mixture of HCl,H₂O₂ and de-ionized water having a concentration ratio of HCl and H₂O₂relative to the de-ionized water of about 0.25 percent to about 10percent; and a mixture of HCl and de-ionized water having aconcentration ratio of HCl relative to the de-ionized water of about0.25 percent to about 10 percent. The method also includes quenching theetched transparent conductive oxide layer by exposing the transparentconductive oxide layer to a basic solution having a pH of greater thanabout 10 and rinsing the quenched transparent conductive oxide layerwith de-ionized water. The method also includes annealing the rinsedtransparent conductive oxide layer at a first temperature and thermalquenching the annealed transparent conductive oxide layer at a secondtemperature lower than the first temperature in a substantially oxygenfree environment.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 depicts a schematic cross-sectional view of one embodiment of aprocess chamber in accordance with the invention.

FIG. 2 depicts a schematic cross-sectional view of another embodiment ofa process chamber in accordance with the invention.

FIG. 3 depicts an exemplary cross sectional view of a crystallinesilicon-based thin film PV solar cell in accordance with one embodimentof the present invention.

FIG. 4 depicts an exemplary cross sectional view of a tandem junction PVsolar cell in accordance with one embodiment of the present invention.

FIG. 5 is a graph showing the etch rate versus time.

FIG. 6 is a graph showing the etch rate versus time.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements and features of oneembodiment may be beneficially incorporated in other embodiments withoutfurther recitation.

DETAILED DESCRIPTION

Embodiments disclosed herein generally relate to a process of depositinga transparent conductive oxide layer over a substrate. The transparentoxide layer is sometimes deposited onto a substrate for later use in asolar cell device. The transparent conductive oxide layer may bedeposited by a “cold” sputtering process. In other words, during thesputtering process, a plasma is ignited in the processing chamber whichnaturally heats the substrate. No additional heat is provided to thesubstrate during deposition such as from the susceptor. After thetransparent conductive oxide layer is deposited, the substrate may beannealed and etched, in either order, to texture the transparentconductive oxide layer. In order to tailor the shape of the texturing,different wet etch chemistries may be utilized. The different etchchemistries may be used to shape the surface of the transparentconductive oxide and the etch rate.

FIG. 1 illustrates an exemplary reactive sputter process chamber 100suitable for sputter depositing materials according to one embodiment ofthe invention. One example of the process chamber that may be adapted tobenefit from the invention is a PVD process chamber, available fromApplied Materials, Inc., located in Santa Clara, Calif. It iscontemplated that other sputter process chambers, including those fromother manufactures, may be adapted to practice the present invention.

The process chamber 100 includes a chamber body 108 having a processingvolume 118 defined therein. The chamber body 108 has sidewalls 110 and abottom 146. The dimensions of the chamber body 108 and relatedcomponents of the process chamber 100 are not limited and generally areproportionally larger than the size of the substrate 114 to beprocessed. Any substrate size may be processed in a suitably configuredchamber. Examples of suitable substrate sizes include substrates havinga surface area of about 2,000 centimeter square or more, such as about4,000 centimeter square or more, for example about 10,000 centimetersquare or more. In one embodiment, a substrate having a surface area ofabout 50,000 centimeter square or more may be processed.

A chamber lid assembly 104 is mounted on the top of the chamber body108. The chamber body 108 may be fabricated from aluminum or othersuitable materials. A substrate access port 130 is formed through thesidewall 110 of the chamber body 108, facilitating the transfer of asubstrate 114 (i.e., a solar panel, a flat panel display substrate, asemiconductor wafer, or other workpiece) into and out of the processchamber 100. The access port 130 may be coupled to a transfer chamberand/or other chambers of a substrate processing system.

A gas source 128 is coupled to the chamber body 108 to supply processgases into the processing volume 118. In one embodiment, process gasesmay include inert gases, non-reactive gases, and reactive gases.Examples of process gases that may be provided by the gas source 128include, but not limited to, argon gas (Ar), helium (He), nitrogen gas(N₂), oxygen gas (O₂), H₂, NO₂, N₂O and H₂O among others.

A pumping port 150 is formed through the bottom 146 of the chamber body108. A pumping device 152 is coupled to the process volume 118 toevacuate and control the pressure therein. In one embodiment, thepressure level of the process chamber 100 may be maintained at about 1Torr or less. In another embodiment, the pressure level of the processchamber 100 may be maintained at about 10⁻³ Torr or less. In yet anotherembodiment, the pressure level of the process chamber 100 may bemaintained at about 10⁻⁵ Torr to about 10⁻⁷ Torr. In another embodiment,the pressure level of the process chamber 100 may be maintained at about10⁻⁷ Torr or less.

The lid assembly 104 generally includes a target 120 and a ground shieldassembly 126 coupled thereto. The target 120 provides a material sourcethat can be sputtered and deposited onto the surface of the substrate114 during a PVD process. The target 120 or target plate may befabricated from a material utilized for deposition species. A highvoltage power supply, such as a power source 132, is connected to thetarget 120 to facilitate sputtering materials from the target 120. Inone embodiment, the target 120 may be fabricated from a materialcontaining zinc (Zn) metal. In another embodiment, the target 120 may befabricated from materials including a metallic zinc (Zn) containingtarget, zinc alloy, zinc and aluminum alloy and the like. In yet anotherembodiment, the target 120 may be fabricated from materials including azinc containing material and an aluminum containing material. In oneembodiment, the target may be fabricated from a zinc oxide and aluminumoxide material.

In one embodiment, the target 120 is fabricated from a zinc and aluminumalloy having a desired ratio of zinc element to aluminum elementfabricated in the target 120. The aluminum formed in the target 120assists in maintaining the target conductivity at a certain range so asto efficiently enable a uniform sputter process across the targetsurface. The aluminum elements in the target 120 are also believed toincrease film transmittance when sputtered off and deposited onto thesubstrate. In one embodiment, the concentration of the aluminum elementformed in the zinc target 120 is controlled at less than about 5 percentby weight. In the embodiment wherein the target 120 is fabricated fromZnO and Al₂O₃ alloy, the Al₂O₃ dopant concentration in the ZnO target iscontrolled at less than about 3 percent by weight, for example aboutless than 2 percent by weight, such as about less than 0.5 percent byweight, for example, about 0.25 percent by weight.

The target 120 generally includes a peripheral portion 124 and a centralportion 116. The peripheral portion 124 is disposed over the sidewalls110 of the chamber 100. The central portion 116 of the target 120 mayhave a curvature surface slightly extending towards the surface of thesubstrate 114 disposed on a substrate support 138. The spacing betweenthe target 120 and the substrate support 138 is maintained between about50 mm and about 150 mm during processing. It is noted that thedimension, shape, materials, configuration and diameter of the target120 may be varied for specific process or substrate requirements. In oneembodiment, the target 120 may further include a backing plate having acentral portion bonded and/or fabricated from a material desired to besputtered onto the substrate surface. The target 120 may also includeadjacent tiles or material segments that together form the target.

Optionally, the lid assembly 104 may further comprise a magnetronassembly 102 mounted above the target 120 which enhances efficientsputtering materials from the target 120 during processing. Examples ofthe magnetron assembly 102 include a linear magnetron, a serpentinemagnetron, a spiral magnetron, a double-digitated magnetron, arectangularized spiral magnetron, among others. Additionally, the target120 may be a cylindrical, rotatable sputtering target assembly in whichthe magnet assembly is within the inner core of the target assembly.

The ground shield assembly 126 of the lid assembly 104 includes a groundframe 106 and a ground shield 112. The ground shield assembly 126 mayalso include another chamber shield member, a target shield member, adark space shield, and a dark space shield frame. The ground shield 112is coupled to the peripheral portion 124 by the ground frame 106defining an upper processing region 154 below the central portion of thetarget 120 in the process volume 118. The ground frame 106 electricallyinsulates the ground shield 112 from the target 120 while providing aground path to the chamber body 108 of the process chamber 100 throughthe sidewalls 110. The ground shield 112 constrains plasma generatedduring processing within the upper processing region 154 and dislodgestarget source material from the confined central portion 116 of thetarget 120, thereby allowing the dislodged target source to be mainlydeposited on the substrate surface rather than chamber sidewalls 110. Inone embodiment, the ground shield 112 may be formed by one or morework-piece fragments and/or a number of these pieces bonding by asubstrate process, such as welding, gluing, high pressure compression,etc.

A shaft 140 extending through the bottom 146 of the chamber body 108couples to a lift mechanism 144. The lift mechanism 144 is configured tomove the substrate support 138 between a lower transfer position and anupper processing position. A bellows 142 circumscribes the shaft 140 andcouples to the substrate support 138 to provide a flexible sealtherebetween, thereby maintaining vacuum integrity of the chamberprocessing volume 118.

A shadow frame 122 is disposed on the periphery region of the substratesupport 138 and is configured to confine deposition of source materialsputtered from the target 120 to a desired portion of the substratesurface. A chamber shield 136 may be disposed on the inner wall of thechamber body 108 and have a lip 156 extending inward to the processingvolume 118 configured to support the shadow frame 122 disposed aroundthe substrate support 138. As the substrate support 138 is raised to theupper position for processing, an outer edge of the substrate 114disposed on the substrate support 138 is engaged by the shadow frame 122and the shadow frame 122 is lifted up and spaced away from the chambershield 136. When the substrate support 138 is lowered to the transferposition adjacent to the substrate transfer port 130, the shadow frame122 is set back on the chamber shield 136. Lift pins (not shown) areselectively moved through the substrate support 138 to list thesubstrate 114 above the substrate support 138 to facilitate access tothe substrate 114 by a transfer robot or other suitable transfermechanism.

A controller 148 is coupled to the process chamber 100. The controller148 includes a central processing unit (CPU) 160, a memory 158, andsupport circuits 162. The controller 148 is utilized to control theprocess sequence, regulating the gas flows from the gas source 128 intothe chamber 100 and controlling ion bombardment of the target 120. TheCPU 160 may be of any form of a general purpose computer processor thatcan be used in an industrial setting. The software routines can bestored in the memory 158, such as random access memory, read onlymemory, floppy or hard disk drive, or other form of digital storage. Thesupport circuits 162 are conventionally coupled to the CPU 160 and maycomprise cache, clock circuits, input/output subsystems, power supplies,and the like. The software routines, when executed by the CPU 160,transform the CPU into a specific purpose computer (controller) 148 thatcontrols the process chamber 100 such that the processes are performedin accordance with the present invention. The software routines may alsobe stored and/or executed by a second controller (not shown) that islocated remotely from the chamber 100.

During processing, the material is sputtered from the target 120 anddeposited on the surface of the substrate 114. The target 120 and thesubstrate support 138 are biased relative to each other by the powersource 132 to maintain a plasma formed from the process gases suppliedby the gas source 128. The ions from the plasma are accelerated towardand strike the target 120, causing target material to be dislodged fromthe target 120. The dislodged target material and process gases forms alayer on the substrate 114 with desired compositions. In some in-lineembodiments, the sputtering target 120 may be biased relative to afloating anode such as the substrate 114, the chamber wall, anothertarget or even another electrode.

FIG. 2 illustrates another exemplary reactive sputter process chamber200 suitable for sputter depositing materials according to oneembodiment of the invention. One example of a process chamber that maybe adapted to benefit from the invention is a PVD process chamber,available from Applied Materials, Inc., located in Santa Clara, Calif.It is contemplated that other sputter process chambers, including thosefrom other manufactures, may be adapted to practice the presentinvention.

The processing chamber 200 includes a top wall 204, a bottom wall 202, afront wall 206 and a back wall 208, enclosing an interior processingregion 240 within the process chamber 200. At least one of the walls202, 204, 206, 208 is electrically grounded. The front wall 206 includesa front substrate transfer port 218 and the back wall 208 includes aback substrate transfer port 232 that facilitate substrate entry andexit from the processing chamber 200. The front transfer port 218 andthe back transfer port 232 may be slit valves or other suitable sealabledoors that can maintain vacuum within the processing chamber 200. Thetransfer ports 218, 232 may be coupled to a transfer chamber, load lockchamber and/or other chambers of a substrate processing system.

One or more PVD targets 220 may be mounted to the top wall 204 toprovide a material source that can be sputtered from the target 220 anddeposited onto the surface of the substrate 250 during a PVD process.The target 220 may be fabricated from a material utilized for depositionspecies. A high voltage power supply, such as a power source 230, isconnected to the target 220 to facilitate sputtering materials from thetarget 220. In one embodiment, the target 220 may be fabricated from amaterial containing zinc (Zn) metal. In another embodiment, the target220 may be fabricated from materials including metallic zinc (Zn), zincalloy, zinc oxide and the like. Different dopant materials, such asboron containing materials, titanium containing materials, tantalumcontaining materials, tungsten containing materials, aluminum containingmaterials, gallium containing materials, indium containing materials,and the like, may be doped into a zinc containing base material to forma target with a desired dopant concentration. In one embodiment, thedopant materials may include one or more of boron containing materials,titanium containing materials, tantalum containing materials, aluminumcontaining materials, tungsten containing materials, gallium containingmaterials, indium containing materials, alloys thereof, combinationsthereof and the like. In one embodiment, the target 220 may befabricated from a zinc oxide material having dopants, such as, titaniumoxide, tantalum oxide, tungsten oxide, aluminum oxide, aluminum metal,boron oxide, gallium, indium, and the like, doped therein. In oneembodiment, the dopant concentration in the zinc containing materialcomprising the target 220 is controlled to less than about 10 percent byweight.

In one embodiment, the target 220 is fabricated from a zinc and aluminumalloy having a desired ratio of zinc element to aluminum element. Thealuminum elements comprising the target 220 assists in maintaining thetarget conductivity within a desired range so as to efficiently enable auniform sputter process across the target surface. The aluminum elementsin the target 220 are also believed to increase film transmittance whensputtered off and deposited onto the substrate 250. In one embodiment,the concentration of the aluminum element comprising the zinc target 220is controlled to less than about 5 percent by weight. In embodimentswherein the target 220 is fabricated from ZnO and Al₂O₃ alloy, the Al₂O₃dopant concentration in the ZnO base target material is controlled toless than about 2 percent by weight, such as less than 0.5 percent byweight, for example, about 0.25 percent by weight.

Optionally, a magnetron assembly (not shown) may be optionally mountedabove the target 220 which enhances efficient sputtering materials fromthe target 220 during processing. Examples of the magnetron assemblyinclude a linear magnetron, a serpentine magnetron, a spiral magnetron,a double-digitated magnetron, a rectangularized spiral magnetron, amongothers.

A gas source 228 supplies process gases into the processing volume 240through a gas supply inlet 226 formed through the top wall 204 and/orother wall of the chamber 200. In one embodiment, process gases mayinclude inert gases, non-reactive gases, and reactive gases. Examples ofprocess gases that may be provided by the gas source 228 include, butare not limited to, argon gas (Ar), helium (He), nitrogen gas (N₂),oxygen gas (O₂), H₂, NO₂, N₂O and H₂O among others. It is noted that thelocation, number and distribution of the gas source 228 and the gassupply inlet 226 may be varied and selected according to differentdesigns and configurations of the specific processing chamber 200.

A pumping device 242 is coupled to the process volume 240 to evacuateand control the pressure therein. In one embodiment, the pressure levelof the interior processing region 240 of the process chamber 200 may bemaintained at about 1 Torr or less. In another embodiment, the pressurelevel within the process chamber 100 may be maintained at about 10⁻³Torr or less. In yet another embodiment, the pressure level within theprocess chamber 200 may be maintained at about 10⁻⁵ Torr to about 10⁻⁷Torr. In another embodiment, the pressure level of the process chamber100 may be maintained at about 10⁻⁷ Torr or less.

A substrate carrier system 252 is disposed in the interior processingregion 240 to carry and convey a plurality of substrates 250 disposed inthe processing chamber 200. In one embodiment, the substrate carriersystem 252 is disposed on the bottom wall 202 of the chamber 200. Thesubstrate carrier system 252 includes a plurality of cover panels 214disposed among a plurality of rollers 212. The rollers 212 may bepositioned in a spaced-apart relationship. The rollers 212 may beactuated by actuating device (not shown) to rotate the rollers 212 aboutan axis 264 fixedly disposed in the processing chamber 200. The rollers212 may be rotated clockwise or counter-clockwise to advance (a forwarddirection shown by arrow 216 a) or backward (a backward direction shownby arrow 216 b) the substrates 250 disposed thereon. As the rollers 212rotate, the substrate 250 is advanced over the cover panels 214. In oneembodiment, the rollers 212 may be fabricated from a metallic material,such as Al, Cu, stainless steel, or metallic alloys, among others.

A top portion of the rollers 212 is exposed to the processing region 240between the cover panels 214, thus defining a substrate support planethat supports the substrate 250 above the cover panels 214. Duringprocessing, the substrates 250 enter the processing chamber 200 throughthe back access port 232. One or more of the rollers 212 are actuated torotate, thereby advancing the substrate 250 across the rollers 212 inthe forward direction 216 a through the processing region 240 fordeposition. As the substrate 250 advances, the material sputtered fromthe target 220 falls down and deposits on the substrate 250 to form aTCO layer with desired film properties. As the substrate 250 continuesto advance, the materials sputtered from different targets 220 areconsecutively deposited on the substrate surface, thereby forming adesired layer of TCO film on the substrate surface.

In order to deposit the TCO layer on the substrate 250 with highquality, an insulating member 210 electrically isolates the rollers 212from ground. The insulating member 210 supports the rollers 212 whileinterrupting the electrical path between the rollers 212 and a groundedsurface, such as the processing chamber 200. As the rollers 212 areinsulated from ground, the substrate 250 supported thereon is maintainedin an electrically floating position, thereby assisting accumulatingions, charges, and species from the plasma on the substrate surface. Theaccumulation of the ions and plasma on the substrate surface helpsretain reactive species on the substrate surface and allows the activespecies to have sufficient time to pack atoms on the substrate surface,thereby improving the quality of the deposited TCO layer, such asproviding high film density. Accordingly, unwanted defects, such asvoids or irregular atoms/grain arrangement may be reduced and/oreliminated, thereby providing a TCO layer having desirable high filmdensity and low film resistivity.

In one embodiment, the insulating mechanism 210 may be in the form of aninsulating pad fabricated from an insulating material, such as rubber,glass, polymer, plastic, and polyphenylene sulfide (PPS),polyetheretherketone (PEEK) or any other suitable insulating materialsthat can provide insulation to the rollers to the bottom wall 202 of theprocessing chamber 200. In one embodiment, the insulating pad 210 is anon-conductive material, such as polyphenylene sulfide (PPS),polyetheretherketone (PEEK), or the like.

A controller 248 is coupled to the process chamber 200. The controller248 includes a central processing unit (CPU) 260, a memory 258, andsupport circuits 262. The controller 248 is utilized to control theprocess sequence, regulating the gas flows from the gas source 228 intothe chamber 200 and controlling ion bombardment of the target 220. TheCPU 260 may be of any form of a general purpose computer processor thatcan be used in an industrial setting. The software routines can bestored in the memory 258, such as random access memory, read onlymemory, floppy or hard disk drive, or other form of digital storage. Thesupport circuits 262 are conventionally coupled to the CPU 260 and maycomprise cache, clock circuits, input/output subsystems, power supplies,and the like. The software routines, when executed by the CPU 260,transform the CPU 260 into a specific purpose computer (controller) 248that controls the process chamber 200 such that the processes areperformed in accordance with the present invention. The softwareroutines may also be stored and/or executed by a second controller (notshown) that is located remotely from the chamber 200.

During processing, as the substrate 250 is advanced by the roller 212,the material is sputtered from the target 220 and deposited on thesurface of the substrate 250. The target 220 is biased by the powersource 230 to maintain a plasma 222 formed from the process gasessupplied by the gas source 228 and biased toward the substrate surface(as shown by arrows 224). The ions from the plasma are acceleratedtoward and strike the target 220, causing target material to bedislodged from the target 220. The dislodged target material and processgases form a layer on the substrate 214 with a desired composition.

FIG. 3 depicts an exemplary cross sectional view of an amorphoussilicon-based thin film PV solar cell 300 in accordance with oneembodiment of the present invention. The amorphous silicon-based thinfilm PV solar cell 300 includes a substrate 318. The substrate 318 maybe thin sheet of metal, plastic, organic material, silicon, glass,quartz, or polymer, or other suitable material. In one embodiment, thesubstrate 318 is a transparent substrate. The substrate 318 may have asurface area greater than about 1 square meters, such as greater thanabout 2 square meters. Alternatively, the thin film PV solar cell 300may also be fabricated as polycrystalline, microcrystalline or othertype of silicon-based thin films as needed. It is to be understood thatthe substrate 318 may be referred to as a ‘superstrate’ in which thesolar cell is fabricated from the top down. During fabrication, thesubstrate 318 is typically refereed to as a substrate, but then referredto as a ‘superstrate’ once the final product is flipped over to face thesubstrate 318 towards the sun. When the final configuration of the solarcell 300 has the substrate facing the sun, the substrate 318 maycomprise a transparent material. When the solar cell 300 is fabricatedsuch that the substrate is opposite the sun, then other materials may beutilized as discussed above.

A photoelectric conversion unit 314 is formed on a transparentconductive layer, such as a TCO layer 302, disposed on the substrate318. The photoelectric conversion unit 314 includes a p-typesemiconductor layer 304, a n-type semiconductor layer 308, and anintrinsic type (i-type) semiconductor layer 306 sandwiched therebetweenas a photoelectric conversion layer. An optional dielectric layer (notshown) may be disposed between the substrate 314 and the TCO layer 302as needed. In one embodiment, the optional dielectric layer may be asilicon layer including amorphous or polysilicon, SiON, SiN, SiC, SiOC,silicon oxide (SiO₂) layer, doped silicon layer, or any suitable siliconcontaining layer. In another embodiment, the optional dielectric layermay be a titanium based layer such as titanium oxide to act as a barrierto impurities that may be present in the substrate 318.

The p-type and n-type semiconductor layers 304, 308 may be silicon basedmaterials doped by an element selected either from group III or V. Agroup III element doped silicon film is referred to as a p-type siliconfilm, while a group V element doped silicon film is referred to as an-type silicon film. In one embodiment, the n-type semiconductor layer308 may be a phosphorus doped silicon film and the p-type semiconductorlayer 304 may be a boron doped silicon film. The doped silicon films304, 308 include an amorphous silicon film (a-Si), a polycrystallinefilm (poly-Si), and a microcrystalline film (pc-Si) with a totalthickness between around 5 nm and about 50 nm. Alternatively, the dopedelement in semiconductor layers 304, 308 may be selected to meet devicerequirements of the PV solar cell 300. The n-type and p-typesemiconductor layers 304, 308 may be deposited by a CVD process or othersuitable deposition process.

The i-type semiconductor layer 306 is a non-doped type silicon basedfilm. The i-type semiconductor layer 306 may be deposited under processcondition controlled to provide film properties having improvedphotoelectric conversion efficiency. In one embodiment, the i-typesemiconductor layer 306 may be fabricated from i-type polycrystallinesilicon (poly-Si), i-type microcrystalline silicon film (μc-Si),amorphous silicon (a-Si), or hydrogenated amorphous silicon (a-Si:H).

After the photoelectric conversion unit 314 is formed on the TCO layer302, a back reflector 316 is formed on the photoelectric conversion unit314. In one embodiment, the back reflector 316 may be formed by astacked film that includes a TCO layer 310, and a conductive layer 312.The conductive layer 312 may be at least one of Ti, Cr, Al, Ag, Au, Cu,Pt, or their alloys. The TCO layer 310 may be fabricated from a materialsimilar to the TCO layer 302 formed on the substrate 318. The TCO layers302, 310 may be fabricated from a selected group consisting of tin oxide(SnO₂), indium tin oxide (ITO), zinc oxide (ZnO), or combinationsthereof. In one exemplary embodiment, the TCO layers 302, 310 may befabricated from a ZnO layer having a desired Al₂O₃ dopant concentrationformed in the ZnO layer.

In embodiments depicted in FIG. 3, at least one of the TCO layers 302,310 is fabricated by a sputter deposition process of the presentinvention. The sputter deposition process of TCO layers 302, 310 may beperformed in the processing chamber 100, as described in FIG. 1 or theprocess chamber 200, as described in FIG. 2.

FIG. 4 depicts an exemplary cross sectional view of a tandem type PVsolar cell 400 fabricated in accordance with another embodiment of thepresent invention. Tandem type PV solar cell 400 has a similar structureof the PV solar cell 300 including a bottom TCO layer 402 formed on thesubstrate 430 and a first photoelectric conversion unit 422 formed onthe TCO layer 402. The first photoelectric conversion unit 422 may beμc-Si based, poly-silicon or amorphous based photoelectric conversionunit as described with reference to the photoelectric conversion unit314 of FIG. 3. An intermediate layer 410 may be formed between the firstphotoelectric conversion unit 422 and a second photoelectric conversionunit 424. The intermediate layer 410 may be a TCO layer sputterdeposited. The combination of the first underlying conversion unit 422and the second photoelectric conversion unit 424 as depicted in FIG. 4increases the overall photoelectric conversion efficiency.

The second photoelectric conversion unit 424 may be pc-Si based,polysilicon or amorphous based, and have an μc-Si film as the i-typesemiconductor layer 414 sandwiched between a p-type semiconductor layer412 and a n-type semiconductor layer 416. A back reflector 426 isdisposed on the second photoelectric conversion unit 424. The backreflector 426 may be similar to back reflector 316 as described withreference to FIG. 3. The back reflector 426 may comprise a conductivelayer 420 formed on a top TCO layer 418. The materials of the conductivelayer 420 and the TCO layer 418 may be similar to the conductive layer312 and TCO layer 310 as described with reference to FIG. 3.

Sputter Deposition of TCO

There are numerous methods to deposit the TCO layer. One method ofdeposition involves utilizing chemical vapor deposition (CVD). CVD is adeposition method that is performed at a very high depositiontemperature such as 300 degrees Celsius and above. The high temperatureenables the technician to control the texture of the TCO and thus, thelight scattering. Another deposition method involves sputtering.Sputtering is beneficial because it obtains a film with low sheetresistance, obtains high transmittance and mobility, and obtains aparticular haze. However, in order to obtain the same film propertiesand the same texturing results as CVD, the temperature of the PVDprocess is performed at an elevated temperature such as about 300degrees Celsius to about 400 degrees Celsius. It is difficult to controlan in-line film process with a uniform temperature, and it is difficultto have reliable hardware and tool ownership is expensive. The highertemperature enables the technician to control the texture of the TCOlayer. The higher temperature is achieved by heating the substrate inaddition to heating the substrate with the plasma generated during theprocess. In one embodiment, the substrate may comprise glass. In anotherembodiment, the substrate may comprise soda lime glass. Whenever thesubstrate contains a material that may diffuse into the other layers, itmay be beneficial to deposit a dielectric diffusion barrier layer (suchas a dielectric sodium diffusion barrier layer) onto the substrate.

It has surprisingly been found that acceptable film properties andtexturing results can be achieved by a ‘cold’ PVD process followed by anannealing process and an etching process. The PVD process discussedbelow may be accomplished without providing any additional heatingbeyond the heating that results from the plasma generated in thechamber. Thus, the process is considered to be a ‘cold’ process becausethe substrate is not actively heated. In one embodiment, the process maybe performed at room temperature. In another embodiment, the processtemperature may be between about 23 degrees Celsius to about 30 degreesCelsius. For large area substrates, controlling the temperature of thesubstrate from the center to the edge can be a challenge when supplyingheat to the substrate through the susceptor. For example, thetemperature at the center may be different than the temperature at theedge which could lead to an unacceptable deposition on the substrate. Ifthe film deposited is unacceptable, the entire substrate is scrapped,which can be costly. An additional problem with the ‘hot’ method wherebythe substrate is heated with a heater in the susceptor is that thethroughput is slower than the ‘cold’ process because the substrate isheated and then rapidly cooled. Therefore, when the plasma is the onlyheating for the substrate, the variable of heating through the susceptoris removed and a level of uncertainty is removed. An additional benefitof the ‘cold’ PVD process is that a more uniform grain structure in thedeposited film results as compared to the ‘hot’ PVD process. Naturally,both the ‘hot’ and ‘cold’ PVD processes may be performed under vacuum.After the TCO has been deposited by ‘cold’ PVD, the substrate is thenannealed and etched to texture the TCO. As will be discussed belowdepending upon the desired end product, the annealing and the etchingmay be performed in any order.

Some of the advantages to utilizing the ‘cold’ PVD approach followed bythe annealing and etching is that the hardware systems are simpler, thehardware systems are more reliable, the hardware systems are more costeffective, that is more control of the TCO film uniformity across largearea substrates for the room temperature PVD process, the system cost ofownership can be much lower and the deposition is possible in existinghigh-volume, in-line glass coating systems.

In order to deposit the TCO, a substrate may be placed into a sputterprocess chamber for deposition of the TCO layer onto the substrate. Aprocess gas mixture is supplied into the sputter process chamber. Theprocess gas mixture supplied into the sputter process chamber bombardsthe source material from the target and reacts with the sputteredmaterial to form the desired TCO layer on the substrate surface. In oneembodiment, the gas mixture may include reactive gas, non-reactive gas,and the like. Examples of non-reactive gas include, but are not limitedto, inert gas, such as Ar, He, Xe, and Kr, or other suitable gases.Examples of reactive gas include, but not limited to, O₂, N₂, N₂O, NO₂,H₂, NH₃, H₂O, among others. Non-reactive gases may be supplied when thesputtering process is an RF, DC or AC sputtering process in which thesputtering target comprises the TCO material to be deposited such asZnO. When the sputtering process is a reactive sputtering process, thesputtering target may comprise the metal for the TCO, such as zinc,which reacts with the reactive gas to deposit ZnO on the substrate.

In one embodiment, the argon (Ar) gas supplied into the sputter processchamber assists in bombarding the target to sputter materials from thetarget surface. The sputtered materials from the target react with thereactive gas in the sputter process chamber, thereby forming a TCO layerhaving desired film properties on the substrate. The TCO layer formed ata different location of the photoelectric conversion unit may havedifferent film properties to achieve different current conversionefficiency requirements. For example, a bottom TCO layer may requirefilm properties, such as relatively high textured surface, hightransparency, and high conductivity. An upper TCO layer may require hightransparency as well, however, any surface texturing is much less thanthat of the bottom TCO layer. The gas mixture and/or other processparameters may be varied during the sputtering deposition process,thereby creating the TCO layer with desired film properties fordifferent film quality requirements. The texturing process will bedescribed below.

In one particular embodiment, the process gas mixture supplied into thesputter process chamber includes at least one of Ar, O₂ or H₂. In oneembodiment, the O₂ gas may be supplied at a flow rate between about 0sccm and about 100 sccm, such as between about 5 sccm and about 30 sccm,for example between about 5 sccm and about 15 sccm. The Ar gas may besupplied into the processing chamber 100 at a flow rate between about150 sccm and between 500 sccm. The H₂ gas may be supplied into theprocessing chamber 100 at a flow rate between about 0 sccm and between100 sccm, such as between about 5 sccm and about 30 sccm. Alternatively,O₂ gas flow may be controlled at a flow rate per total flow rate belowabout 0.1 percent of the total gas flow rate. H₂ gas flow may becontrolled at a flow rate per total flow rate below about 0.5 percent ofthe total gas flow rate.

In the embodiment wherein the gas mixture supplied into the processchamber includes Ar and O₂ gas, the Ar gas flow rate supplied in the gasmixture is controlled at between about 90 percent by volume to less than100 percent by volume and the oxygen gas flow rate is controlled toabout less than 10 percent by volume. In the embodiment wherein the gasmixture supplied into the process chamber includes Ar, O₂ and H₂ gas,the Ar gas flow rate supplied in the gas mixture is controlled atbetween about 80 percent by volume to less than 100 percent by volume,the oxygen gas flow rate is controlled about less than 10 percent byvolume, and the hydrogen gas flow rate is also controlled to less thanthe flammability limit such as at about less than 5 percent by volume.

As different gas mixtures supplied into the process chamber may providedifferent source ions that may be reacted with the source materialsputtered off the target, by supplying different gas species in the gasmixture, a control of the film properties of the TCO layer may beobtained. For example, a greater amount of oxygen gas supplied in thegas mixture may result in a TCO layer having a higher quality of oxygenelements formed in the resultant TCO layer. Accordingly, by controllingthe amount of reactive gas along with different selection of targetsused during sputtering, a TCO layer having tailored film properties maybe obtained.

After the processing gas is introduced to the chamber, RF power issupplied to the target to sputter the source material from the targetwhich reacts with the gas mixture. It is to be understood that whilereference is made to an RF reactive sputtering process, the process ofdepositing the TCO may be accomplished utilizing a DC or AC reactive ornon-reactive sputtering process. Additionally, it is to be understoodthat the RF reactive sputtering process may be accomplished byintroducing reactive gases in addition to inert gases into theprocessing chamber.

As a high voltage power is supplied to the zinc (Zn) and aluminum (Al)alloy target, the metal zinc and aluminum source material is sputteredfrom the target. The bias power applied between the target and thesubstrate support maintains a plasma formed from the gas mixture in theprocess chamber. The ions mainly from the gas mixture in the plasmabombard and sputter off material from the target. The reactive gasesreact with the growing sputtered film to form a layer with desiredcomposition on the substrate. In one embodiment, a metal alloy targetmade of zinc (Zn) and aluminum (Al) metal alloy is utilized as a sourcematerial of the target for sputter process. The ratio of Al metalincluded in the Zn target is controlled at about less than 3 percent byweight, such as less than 2 percent by weight, such as about less than0.5 percent by weight, for example, about 0.25 percent by weight. Inanother embodiment, a metal alloy target made of zinc oxide (ZnO) andaluminum oxide (Al₂O₃) metal alloy is utilized as a source material ofthe target for sputter process. The ratio of Al₂O₃ included in the ZnOtarget is controlled at between about less than 3 percent by weight, forexample about less than 2 percent by weight, such as about less than 0.5percent by weight, for example, about 0.25 percent by weight.

In the embodiment wherein the target is made of zinc (Zn) and aluminum(Al) metals, the gas mixture supplied for sputtering may include argonand oxygen gas. The argon gas is used to bombard and sputter the target,and the oxygen ions dissociated from the O₂ gas mixture reacts with thezinc and aluminum ions sputtered from the target, forming a zinc oxide(ZnO) and aluminum oxide (Al₂O₃) containing TCO layer on the substrate.The RF, DC or AC power is applied to the target during processing. Inthe embodiment wherein the target is fabricated from ZnO having Al₂O₃doped therein, the gas mixture used to bombard the target may includeargon but may or may not include O₂ gas. In this embodiment, the oxygengas may be optionally eliminated as the target provides the oxygenelements that are deposited in the TCO layer. In some embodiments, thehydrogen gas may be used in the gas mixture to assist in bombarding andreacting with the source material from the target regardless of thematerials of the target.

In one embodiment, a RF power of between about 100 Watts and about 60000Watts may be supplied to the target. Alternatively, the RF power may becontrolled by RF power density supplied between about 0.15 Watts percentimeter square and about 15 Watts per centimeter square, for example,about 4 Watts per centimeter square and about 8 Watts per centimetersquare. Alternatively, the DC power may be supplied between about 0.15Watts per centimeter square and about 15 Watts per centimeter square. Inone embodiment, a mid-frequency AC electrical bias may be applied to azinc target to sputter the zinc which will react with oxygen to depositzinc oxide on the substrate. In one embodiment, the mid-frequency ACelectrical bias may be between about 2 kW/meter target length to about17 kW/meter target length.

Several process parameters may be regulated. In one embodiment, apressure of the gas mixture in the process chamber is regulated betweenabout 2 mTorr and about 10 mTorr. The substrate temperature may bemaintained between about 25 degrees Celsius and about 100 degreesCelsius. The processing time may be processed at a predeterminedprocessing period or after a desired thickness of the layer is depositedon the substrate. In one embodiment, the process time may be processedat between about 30 seconds and about 400 seconds. In one embodiment,the thickness of the TCO layer is between about 5,000 Å and about 10,000Å. In the embodiment wherein a substrate with different dimension isdesired to be processed, process temperature, pressure and spacingconfigured in a process chamber with different dimension do not changein accordance with a change in substrate and/or chamber size.

During the deposition process, as the ions dissociated from the gasmixture react with sputtered off material from the target, a TCO layerwith desired composition is therefore formed on the substrate surface.In one embodiment, the TCO layer as deposited is a ZnO layer having adesired amount of aluminum oxide dopant formed therein. It is believedthat the TCO layer having a desired amount of Al₂O₃ dopant formed in theZnO layer can efficiently improve current conversion efficiency of thephotoelectric conversion unit. The aluminum elements formed in the TCOlayer may provide higher film conductivity, thereby assisting carryinggreater amount of current in the TCO layer. Additionally, it is believedthat higher amount of oxygen elements formed in the TCO layer increasesfilm transmittance that allows greater amount of current generated inthe photoelectric conversion unit. Furthermore, a high film transparencyis desired to maximize the light transmitting efficiency. Accordingly,by controlling a desired amount of aluminum oxide formed in the zinccontaining layer, the TCO layer having desired film properties, such ashigh transmittance and high current conversion efficiency, may beobtained.

In one embodiment, the oxygen source formed in the TCO layer may beprovided from the gas mixture supplied into the process chamber duringsputter process. Alternatively, the oxygen source may be provided from aselected target having metal oxide alloy prefabricated in the target sothat when sputtering, both metallic and oxygen ions may be sputtered offthe target and deposited on the substrate surface. In the embodimentwherein the selected target is fabricated from a zinc and aluminum metalalloy, a gas mixture including argon, oxygen may be used to provideoxygen ions, when dissociated, to react with the zinc and aluminum ionssputtered from the target, forming zinc oxide layer having desiredconcentration of aluminum oxide on the substrate. In the embodimentwherein the selected target is fabricated from zinc oxide and aluminumoxide, a gas mixture including argon gas may be used. The oxygen gas maybe optionally supplied in the gas mixture. The hydrogen gas may beoptionally supplied in both cases.

As discussed above, a TCO layer having a desired amount of Al₂O₃ dopantformed in the ZnO layer may improve the film conductivity and filmtransparency. The Al₂O₃ dopant source may be provided from the targetduring processing. In one embodiment, the ratio of Al₂O₃ included in theZnO target is controlled at between about less than 3 percent, forexample about less than 2 percent by weight, such as about less than 0.5percent by weight, for example, about 0.25 percent by weight. In oneembodiment, the lower the dopant concentration of Al₂O₃ formed in theZnO target, a relatively higher amount of oxygen gas may be supplied inthe gas mixture during sputtering to maintain a desired transmittanceformed in the TCO layer. For example, if the ratio of Al₂O₃ doped in theZnO target is about 0.5 percent by weight, the gas mixture may have anoxygen gas flow rate about 5 percent by volume and argon gas flow rateabout 95 percent by volume. However, if the ratio of Al₂O₃ doped in theZnO target is as low as about 0.25 percent by weight, the gas mixturemay have a higher oxygen gas flow rate about 7-8 percent by volume andlower argon gas flow rate about 92-93 percent by volume. Since bothoxygen elements and Al₂O₃ elements formed in the TCO layer are believedto increase film transmittance, when a lower dopant concentration ofAl₂O₃ target is used, a higher oxygen gas in the gas mixture may be usedto compensate the lower dopant concentration of Al₂O₃ formed in thetarget. In some embodiments, hydrogen gas or water vapor may also beutilized to increase the resultant film transmittance or enhance otherfilm properties. In one embodiment, the TCO layer has an Al₂O₃ dopantconcentration between about 0.25 percent and about 3 percent in a ZnObased layer.

In operation, the incident light provided by the environment is suppliedto the PV solar cell. The photoelectric conversion unit in the PV solarcell absorbs the light energy and converts the light energy intoelectrical energy by operation of the p-i-n junctions formed in thephotoelectric conversion unit, thereby generating electricity or energy.Alternatively, the PV solar cell may be fabricated or deposited in areversed order. For example, the substrate may be disposed over the backreflector.

While the TCO has been described as being ZnO, it is to be understoodthat other materials may be used as the TCO such as SnO. Additionally, adopant need not be present. However, if a dopant is present, the dopantneed not be aluminum, but rather, may be any of a number of otherdopants such as titanium, tantalum, gallium, cadmium, boron, indium,fluorine, and tin.

Once the TCO has been deposited, the post deposition treatment of theTCO occurs in order to improve film properties and obtain the desiredtexture of the TCO. The post deposition treatment involves annealing thesubstrate and etching the TCO. Depending upon the desired final product,the annealing and the etching may be reversed such that the etchingoccurs before the annealing. Additionally, after the texture of the TCOhas been measured, additional annealing and/or etching may occur ifnecessary. In one embodiment, the annealing and the sputtering may occurin the same chamber. Performing the annealing and sputtering in the samechamber may be beneficial if the annealing is performed after thesputtering and before the texturing. In another embodiment, theannealing may occur in a separate chamber from the sputtering. If thetexturing is performed before the annealing, it may be beneficial tohave a separate annealing chamber.

Vacuum Annealing

In one embodiment for the annealing, the substrate (and hence the TCOdeposited thereover) is heated to an annealing temperature for apredetermined period of time. In one embodiment, the annealingtemperature may be between about 250 degrees Celsius to about 600degrees Celsius. In one embodiment, the annealing may occur for about 15minutes to about 60 minutes. In another embodiment, the annealing mayoccur for 5 minutes or more at ambient pressure. The annealing relaxesout the defects in the TCO and thus improves the conductivity andtransmission of the TCO. Additionally, the annealing may densify thedeposited TCO and causes the grains to arrange into a more uniformstructure. The annealing may occur in numerous environments. In onepreferred embodiment, the annealing occurs in an environment of N₂ andH₂ forming gas. In another preferred embodiment, the annealing occurs inan environment of Ar and H₂. In another preferred embodiment, theannealing occurs in an environment of N₂. In another preferredembodiment, the annealing occurs in an environment of Ar. It iscontemplated that other gases may be used as well such as ammonia, N₂O,NO, NO₂, hydrazine, O₂, CO, CO₂, water vapor or combinations thereof.The annealing may increase the transmittance of the TCO to greater than80 percent and decrease the Rs to about 10 ohm/sq or lower.

The annealing process may have several steps. One embodiment of anannealing process is described here. The first step involves evacuatingthe processing chamber. During the evacuation, the substrate is notheated and no gas is provided to the processing chamber. The evacuatingmay take about 10-20 minutes and lower the pressure in the chamber fromabout atmospheric pressure to about 35 to about 50 mTorr.

Thereafter, the initial heat-up proceeds. To perform the initialheat-up, the susceptor heater is turned on and the gas is introduced ata flow rate of about 50 sccm to reduce the pressure to about 2 to about3 mTorr. During the initial heat up, diffusion pumps may be used tomaintain the pressure. The initial heat-up may take from about 30minutes to about 90 minutes.

Then, the annealing begins. The annealing takes about 15 minutes toabout 60 minutes. During the annealing, the gas is provided at a flowrate of about 90 sccm and the chamber is evacuated to maintain apressure of about 150 mTorr.

After the annealing, the substrate is cooled down in a two stage cooldown process. The susceptor heater is turned off and will remain off forthe remainder of the annealing process. In the first stage, which maytake between about 30 minutes to about 60 minutes, the gas is continuedto be provided at about 50 sccm and the pressure is maintained at about2 mTorr to about 3 mTorr. In the second stage, which may take about 60minutes to about 120 minutes, no gas is provided and the chamberpressure is further reduced to about 0.001 mTorr to about 0.005 mTorr.Thereafter, the chamber is vented to atmosphere which takes about 2minutes. The annealing process described here is a vacuum process.Experiments have indicated that atmospheric pressure processes may beeffective provided that the gas environment is properly controlled.Furthermore, it is assumed that the throughput of the annealing processcan be significantly improved by enhancements to the vacuum annealinghardware, or with an atmospheric pressure annealing process.

Atmospheric Annealing

In one embodiment, the annealing may occur at essentially atmosphericpressure. Previously, to make the ZnO with Al₂O₃ doping, a technicianneeded to heat-up the PVD chamber up to 300-400 degrees Celsius toachieve the predetermined film characteristics, such as low sheetresistance, high transmittance mobility, and predetermined haze. Withthis approach, there were a lot of disadvantages due to high processcosts, difficultly to control in-line film formation with a uniformtemperature, difficultly to have reliable hardware, and the high cost oftool ownership.

It has surprisingly been found that a tempering chamber may be utilizedto perform the annealing. The annealing may occur as a two stageannealing process. In the first stage, the substrate with the TCOdeposited thereon is heated in an environment that is substantially atatmospheric pressure. Then, in the second stage, the TCO layer isthermally quenched with a high flow of a gas or gas mixture that is freeof oxygen. During the thermal quenching, the temperature of thesubstrate is lowered. After the thermal quenching, the substrate maythen be exposed to atmosphere.

The advantage of utilizing a cold TCO deposition, followed by textureetch, thermal anneal and thermal quench, as compared to high temperaturedeposition, is that the hardware system utilized is simpler, morereliable, and cost effective. Additionally, there is more control of theTCO film uniformity across large area substrates for room temperaturePVD. The system cost of ownership can be much lower than that for hightemperature processes. The process permits deposition on existinghigh-volume, in-line substrate coating systems.

The thermal treatment or annealing may occur at a temperature of atleast 200 degrees Celsius. In one embodiment, the annealing may occur ata temperature of about 400 degrees Celsius. In another embodiment, theannealing may occur at a temperature of between about 300 degreesCelsius and about 500 degrees Celsius.

The annealing process consists of heat-up, annealing step at the desiredtemperature, and cool down (e.g., thermal quenching). During all stepsit is preferable to have an O₂-free environment to suppress oxidation ofthe TCO film and maintain desirable electrical and optical properties.Examples of suitable gas compositions that may be used include N₂/H₂,Ar/H₂, and N₂. For the N₂/H₂ mixture, the nitrogen may be present in anamount of about 96 percent by volume while the hydrogen may be presentin an amount of about 4 percent by volume. For the argon/hydrogenmixture, the argon may be present in an amount of about 97 percent byvolume while the hydrogen may be present in an amount of about 3 percentby volume. The annealing may occur at near-atmospheric pressures.Because the annealing may occur at near-atmospheric pressures, atempering furnace similar to those used in the production of commercialtempered glass may be used for the annealing process. Typically oncommercial tempered glass tempering furnaces, a high convective flow ofair is used to quench the heated glass. For TCO annealing applications,a high flow of preferably O₂-free gas is be used, and would enablehigher throughput because of the rapid cooling. While the targetpressure is atmospheric pressure for the annealing, due to the high gasflow rate of the O₂-free gas or gas mixture the actual pressure may benominally above atmospheric pressure. It is to be understood that whilean O₂-free environment is preferred, oxygen may be present in smallquantities. For example, the atmosphere in which the anneal may occurmay have a partial pressure of oxygen that is below the partial pressureof oxygen present in air. Additionally or alternatively, a sacrificialprotective layer may be deposited over the TCO prior to annealing topermit the TCO to be annealed in an environment that could include air.

The annealing and thermal quenching may proceed as follows. Thesubstrate may be placed into the annealing chamber. It is to beunderstood that the “annealing chamber” may refer to simply a temperingchamber that is separated from the vacuum sputtering chamber, thesputtering chamber itself when it is used to perform both deposition andannealing, and a chamber of an in-line system. After the substrate is inthe annealing chamber, the temperature of the substrate (andcorrespondingly the TCO) is raised to a temperature within the rangesdiscussed above. During the annealing, the TCO is exposed to asubstantially oxygen free environment at a pressure that may benominally above atmospheric pressure. The oxygen free environmentprevents any further oxidation of the TCO layer. Alternatively, areduced oxygen partial pressure atmosphere may be utilized or aprotective layer may be formed over the TCO prior to annealing or athicker TCO layer may be used with appropriate properties to protect thefilm thereunder. The annealing may occur for a time period of up to 60minutes. Following the annealing, the thermal quenching begins. Thesubstrate may simply remain within the same chamber, be moved to anothersection of the chamber, or moved to a completely separate chamber. Inany event, the TCO is not exposed to air or oxygen between the annealingand thermal quenching processes. Oxygen free gas or gas mixtures aredelivered to the thermal quenching location. The pressure during thethermal quenching may be nominally above atmospheric pressure due to theflow of the gas. During the thermal quenching, the temperature of thesubstrate is reduced to below 200 degrees Celsius. In one embodiment,the temperature of the substrate is reduced to about 100 degreesCelsius. Following the thermal quenching, the TCO may be exposed to anoxygen containing environment such as air.

As mentioned above, the order of the texture etching and the annealingmay be switched based upon optimization with respect to film properties,solar cell performance, and system throughput. For the sequence in whichannealing is performed after etching, annealing is performed on astand-alone separate tool. This tool would be either a vacuum annealingsystem or more preferably an atmospheric pressure system for thepotential of lower cost and higher throughput. If the annealing isperformed first, this can be performed in line by addition of additionalcompartments onto the PVD coater, or in a separate tool.

Etching

The etching may occur to texture the TCO. In one embodiment, the etchingmay comprise a wet etching process. The etching may occur for a timeperiod of between about 10 seconds to about 60 seconds. In oneembodiment, the etching solution may comprise HCl, H₂O₂ and de-ionizedwater in a concentration ratio of HCl and H₂O₂ total solution betweenabout 0.25 percent to about 10 percent. In another embodiment, theetching solution may comprise HCl and de-ionized water where the HCl ispresent in a concentration of about 0.25 percent to about 10 percent ofthe total solution. In another embodiment, the etching solution maycomprise HNO₃ and de-ionized water with a concentration ratio of HNO₃ tototal solution of between about 0.25 percent and about 10 percent. Inanother embodiment, the etching solution may comprise H₂SO₄ andde-ionized water with the H₂SO₄ having a concentration ratio of betweenabout 0.25 percent to about 10 percent of the total solution. For theetching, a nearly completely diffuse transmission or reflection of lightwithout optical loss on the interface is the goal of the texture etch.The texturing is beneficial for light trapping by amorphoussilicon/microcrystalline silicon absorption for long wavelengths becausesilicon absorption is limited by indirect band gap nature. The etchingis beneficial in improving the haze of the TCO. In one embodiment, analuminum doped ZnO having 1% aluminum may have a haze of about 1.36prior to etching, a transmittance of 82 percent, a sheet resistance ofbetween about 5.8 Ω/□ to about 7.6 Ω/□, a resistivity of about 5.9×10⁻⁴Ω-cm, a mobility of 26.2 cm²/V-s and a concentration of 4.08×10²⁰ cm⁻³.

The etching chemistry may be chosen based upon the desired texturing tobe obtained. For example, the different etching chemistries will affectthe shape of the texture. A pyramid or pointy roughness may not bebeneficial because of how the light reflects and refracts through theTCO. A rounded shaped on the surface may be better than a pointy shape.The material of the TCO will also affect the etching rate and how theetching occurs. Thus, the etching chemistry for a ZnO TCO may bedifferent than the etching chemistry for a SnO TCO.

Etch Quenching

After the TCO layer is etched to texture the top surface, the solar cellstructure may then be quenched by exposing the TCO layer to a solutionhaving a pH sufficient to neutralize the etching solution that mayremain on the TCO layer. The quenching may occur for a sufficient timeto prevent further etching by the etching solution, but for only so longas to not being etching the TCO layer with the quenching solution. Afterthe quenching is completed, the solar cell structure may be rinsed inde-ionized water to remove any remaining etching solution or quenchingsolution.

When a TCO layer is deposited, the TCO layer may be textured to improvethe functionality of the solar cell. The etching textures the TCO. Theetching may occur for a time period of between about 20 seconds to about120 seconds. The etching chemistry may be chosen based upon the desiredtexturing to be obtained. Thus, the etching chemistry for a ZnO TCO maybe different than the etching chemistry for a SnO TCO. For example, SnOthat has been pyrolytically CVD deposited may not be wet etched becausethe SnO is textured as it is deposited based upon the depositionconditions. One manner of etching the ZnO TCO layer involves exposingthe deposited ZnO TCO layer to an etchant such as HCl. In oneembodiment, the HCl may be diluted in de-ionized water so that it isabout 0.2 percent to about 5 percent by volume. Other etchants may beused as well. For example, HNO₃, H₂SO₄, H₃PO₄ and combinations thereofare other acids that may be used to etch the ZnO TCO layer.

The primary mechanism of TCO texturing is believed to be etching aroundgrain boundaries resulting in surface texturing. The object of texturingthe TCO layer is to achieve good uniformity of texture and a high hazesurface texture. One of the problems with wet etching is the transitionfrom the wet etching solution to the de-ionized water rinse. The acidcontinues to etch the TCO while the substrate is transferred to thede-ionized water rinsing station from the etch station because etchantremains on the TCO layer. If the pH is low, such as less than 3, theetching continues to occur at the surface boundary. As a result, theuniformity and the texture morphology are affected. FIG. 5 is a graphshowing the etch rate versus time where the etchant continues to etchthe TCO as the TCO is moved to the rinse station. By quenching the TCOwith a higher pH solution before rinsing the TCO, the acid etching maystop. The higher pH solution may be used to counteract and neutralizethe etchant remaining on the TCO. The quenching may be performed withdiluted ammonia hydroxide (0.10 to 6 percent) solution to planarize thetextured surface. By planarizing the textured surface, it is understoodto mean uniformly textured. As used in this specification, planarizingdoes not mean forming a substantially flat surface.

The goal for the final product of the TCO layer is to have a high hazeand a low resistivity. When the TCO is etched, the TCO is textured andthus, the top surface of the TCO may not be perfectly planar which mayincrease the resistivity. However, the etching increases the haze bytexturing the surface to achieve better light trapping and improve solarcell efficiency. A happy balance should be struck between theresistivity and the haze.

As discussed above, once the TCO has been textured, the substrate istransferred to a de-ionized water rinse to remove any remaining etchant.In order to reduce any additional etching of the TCO once the substratehas been removed from the etching location (spray or dip bath forexample) the TCO may be quenched to counteract the effects of theetchant. The quench may be a high pH solution that is used to improvethe TCO texturing process. In one embodiment, the process may proceed asfollows. The TCO, for example ZnO, may be etched with an etchant, thenquenched with a base solution and then rinsed in de-ionized water. Forthe etching, the etching may comprise exposing the TCO to an etchantsuch as HCl, a solution having a pH of between about 1 and about 1.3, ora solution having an acid (such as HCl, HNO₃, H₃PO₄, H₂SO₄ andcombinations thereof) diluted to between about 0.2 percent to about 0.5percent by volume. For the quenching, the basic solution may compriseammonium hydroxide, a basic solution (such as a solution containingammonium hydroxide or KOH) diluted to between about 0.1 percent to about10 percent by volume, or a solution having a pH of between 10 and 11.

The etching results in anisotropic etching of the TCO layer at the grainboundaries. The basic solution counteracts or neutralizes the acidetchant to stop etching of the TCO layer. Other basic solutions may beutilized as an alternative to or in addition to the ammonium hydroxide.For example, KOH may be used as a basic solution. In general, the basicsolution should have a pH of 10 or more.

FIG. 6 is a graph showing the etch rate versus time. In the embodimentshown in FIG. 6, the TCO has been etched in HCl and then quenched inammonia hydroxide. In comparing FIG. 6 to FIG. 5, it is clear that thequenching significantly reduced the amount of etching that occurredafter the etching step was complete and before the rinsing started.

One of the unexpected benefits of quenching is that the haze of the TCOlayer is improved. Additionally, the basic solution, such as ammoniumhydroxide, cleans the surface of the TCO to remove any zinc particlesthat have not reacted with oxygen. The quenching may occur by sprayingthe basic solution onto the TCO, dipping the TCO into the basicsolution, or other effective delivery methods for the quenchingsolution.

It is important that the quenching does not begin to etch the TCO layerbefore the rinsing. If the basic solution begins to etch the TCO, thenthe peaks of the texturized TCO layer may be dulled which may actuallyreduce haze and solar cell efficiency. It has been determined that thequenching should occur for a period of between about 10 seconds andabout 30 seconds. If the quenching occurs for less than about 10seconds, the quenching has a negligible effect and the TCO is etched bythe etchant. If the quenching occurs for more then about 30 seconds,then the basic solution may begin to etch the TCO. The pH of thequenching solution should be 10 or above. For pH values below 10, thebasic solution may not be sufficiently strong to quickly stop theetching. Thus, it has been found that the most effective conditions area pH of greater than 10 and quenching for between 10 seconds and 30seconds. The quenching may occur by dipping the TCO into the solution orby spraying the solution onto the TCO or by other well known methods ofexposing a TCO to a solution.

A TCO that is simply etched without quenching (for example, 30 secondetch in 0.2 volume percent HCl with de-ionized water dip) may have ahaze of about 24 percent while a TCO that is etched with the sameetching conditions but then quenched (for example ammonium hydroxide),the haze is increased to about 28 percent. The mobility may alsoincrease with the use of a quenching step. Thus, the haze may increaseby more than 10 percent and the mobility of the TCO may increase about 2percent.

There are several advantages to utilizing the quenching process afterthe etching process. A post HCl wet etch quench with ammonia hydroxidemay provide a fast etch stop and improve the texture uniformity. A postHCl etch surface treatment with ammonia hydroxide is used to remove postHCl ZnO particles and residues. Ammonia hydroxide etch quench may beused to increase the haze with a minimal sheet resistance increase. Anextended ammonia hydroxide treatment after the HCl etch may be used tomodify the texture morphology and provide texture planarization toimprove ZnO TCO and amorphous silicon film interface.

As mentioned above, the annealing and etching may occur in any order,depending upon the desired ultimate use of the TCO. For example, whenthe TCO will eventually be a part of a tandem junction solar cell, theetching may occur before the annealing. By performing the etching beforethe annealing, the texture roughness of the TCO may be generally higherthan when the annealing occurs first. Because of the greater texture orsurface roughness, the haze may be greater than about 25. A higher hazeis beneficial of obtaining higher solar cell efficiency in a tandemjunction solar cell.

When the TCO is to be used in a single junction solar cell, it may bebeneficial to perform the annealing before the etching. The annealingcould be performed within a hybrid heater system without exposing theTCO to air in between PVD and annealing. Additionally, the haze for thesingle junction solar cell may be about 13 or greater. By performing theannealing before the etching, the throughput of the system may be higherbecause the evacuation step may be eliminated that would otherwise bepresent between an etching and annealing process. It is to be understoodthat while wet etching has been specifically described, it iscontemplated that a dry etching process may be performed instead.Additionally, the PVD, annealing and etching may be performed in asingle cluster tool having all three processing chambers attached to acommon transfer chamber. Alternatively, a single chamber may be utilizedto perform both the PVD and annealing and a separate etching chamber maybe used for texturing. Alternatively, the PVD, annealing and etching maybe performed in an in-line system.

By performing a ‘cold’ PVD process, annealing and etching, a TCO filmmay be formed on a substrate that has the desired haze and efficiency tomeet the needs of solar cell manufacturing. The TCO formed from themethods described herein may also be beneficial for use inelectrochromic technologies.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A method, comprising: forming a transparent conductive oxide layerover a substrate; etching the formed transparent conductive oxide layerto form a roughened surface, the etching comprising exposing the formedtransparent conductive oxide layer to a wet etchant composition selectedfrom the group consisting of: a mixture of HCl, H₂O₂ and de-ionizedwater having a concentration ratio of HCl and H₂O₂ relative to thede-ionized water of about 0.25 percent to about 10 percent; a mixture ofHCl and de-ionized water having a concentration ratio of HCl relative tothe de-ionized water of about 0.25 percent to about 10 percent; amixture of HNO₃ and de-ionized water having a concentration ratio ofHNO₃ relative to the de-ionized water of about 0.25 percent to about 10percent; and a mixture of H₂SO₄ and de-ionized water having aconcentration ratio of H₂SO₄ relative to the de-ionized water of about0.25 percent to about 10 percent; and annealing the etched transparentconductive oxide layer.
 2. The method of claim 1, wherein forming thetransparent conductive oxide layer comprises sputter depositing thetransparent conductive oxide layer.
 3. The method of claim 1, furthercomprising: depositing a p-doped semiconductor layer over the annealedtransparent conductive oxide layer; depositing an intrinsicsemiconductor layer over the p-doped semiconductor layer; and depositingan n-doped semiconductor layer over the intrinsic semiconductor layer.4. The method of claim 1, further comprising: quenching the etchedtransparent conductive oxide layer by exposing the transparentconductive oxide layer to a basic solution having a pH of greater thanabout 10; and rinsing the quenched transparent conductive oxide layerwith de-ionized water.
 5. The method of claim 4, wherein the quenchingcomprises spraying the basic solution onto the transparent conductiveoxide layer or dipping the transparent conductive oxide layer into thebasic solution.
 6. The method of claim 5, wherein the basic solutioncomprises ammonium hydroxide.
 7. The method of claim 5, wherein thebasic solution comprises potassium hydroxide.
 8. The method of claim 5,wherein forming the transparent conductive oxide layer comprises sputterdepositing the transparent conductive oxide layer.
 9. The method ofclaim 1, wherein the annealing comprises: thermal treating thetransparent conductive oxide layer at a first temperature; and thermalquenching the thermally treated transparent conductive oxide layer at asecond temperature lower than the first temperature.
 10. The method ofclaim 9, wherein the thermal quenching comprises reducing thetemperature of the thermally treated transparent conductive oxide layerfrom the first temperature to the second temperature in a substantiallyoxygen free atmosphere.
 11. The method of claim 10, wherein the thermalquenching further comprises exposing the thermally treated transparentconductive oxide layer to a gas selected from the group consisting of:N₂/H₂ mixture; Ar/H₂ mixture; or N₂.
 12. The method of claim 11, whereinforming the transparent conductive oxide layer comprises sputterdepositing the transparent conductive oxide layer.
 13. The method ofclaim 9, wherein the transparent conductive oxide layer is maintained ina substantially oxygen free environment during the thermal treating, thethermal quenching, and during any time between the thermal treating andthermal quenching.
 14. The method of claim 13, wherein forming thetransparent conductive oxide layer comprises sputter depositing thetransparent conductive oxide layer.
 15. The method of claim 9, whereinthe thermal quenching comprises reducing the temperature of thethermally treated transparent conductive oxide layer from the firsttemperature to the second temperature in an environment having a partialpressure of oxygen that is below the partial pressure of oxygen presentin air.
 16. The method of claim 9, wherein the transparent conductiveoxide layer is maintained in an environment having a partial pressure ofoxygen that is below the partial pressure of oxygen present in airduring the thermal treating, the thermal quenching, and during any timebetween the thermal treating and thermal quenching.
 17. The method ofclaim 1, further comprising depositing a sacrificial protective layerover the transparent conductive oxide layer prior to annealing.
 18. Amethod, comprising: sputter depositing a transparent conductive oxidelayer over a substrate; etching the sputter deposited transparentconductive oxide layer to form a roughened surface, the etchingcomprising exposing the sputter deposited transparent conductive oxidelayer to a wet etchant composition selected from the group consistingof: a mixture of HCl, H₂O₂ and de-ionized water having a concentrationratio of HCl and H₂O₂ relative to the de-ionized water of about 0.25percent to about 10 percent; and a mixture of HCl and de-ionized waterhaving a concentration ratio of HCl relative to the de-ionized water ofabout 0.25 percent to about 10 percent; quenching the etched transparentconductive oxide layer by exposing the transparent conductive oxidelayer to a basic solution having a pH of greater than about 10; rinsingthe quenched transparent conductive oxide layer with de-ionized water;annealing the rinsed transparent conductive oxide layer at a firsttemperature; and thermal quenching the annealed transparent conductiveoxide layer at a second temperature lower than the first temperature ina substantially oxygen free environment.
 19. The method of claim 18,wherein the quenching comprises spraying the basic solution onto thetransparent conductive oxide layer or dipping the transparent conductiveoxide layer into the basic solution.
 20. The method of claim 19, whereinthe basic solution comprises ammonium hydroxide.